1. Field of the Invention
This invention pertains to the art of fiber reinforced resin matrix composites, generally of oriented fiber reinforcement. More specifically, a composite material repair system, and method of effecting field repairs, is disclosed.
2. Background of the Invention
Fiber reinforced resin matrix composite materials have found increasing application in fields where high strength and weight savings are twin concerns. Thus, increasingly, aircraft, missiles, spacecraft and support vehicles exhibit a significant percentage of these composite materials in structural or critical elements, such as fuselage structures, as well as non-critical features such as seating and cabin apparatus. These applications, particularly the adoption of composite material in military applications, has created a pressing need for a method of repairing damage to composite materials outside of sophisticated factories and laboratories; in particular, a need to provide a method for on-site repairs at air fields, depots and the like is presented. Current trends indicate that such repair capabilities will be essential to maintain an operative aerospace system in the future.
Any composite material on-site repair system must meet several objectives. Whatever materials are employed in the repair system must have a relatively long shelf life at ambient conditions. Sophisticated refrigeration and low pressure containment systems will not be available for field repairs and are not likely to be available at remote depots. Such apparatus is also susceptible to interruption, which would threaten the integrity of any repair system requiring such apparatus. Repeated destruction and reacquisition of relatively short shelf life products is expensive and impractical.
The repair system must be capable of being cured at relatively low temperatures, very rapidly, without the need for extensive design of repair parts. Generally, necessary temperatures must be below the boiling point of water, 212.degree. F. By the same token, the equipment used in effecting repairs must be relatively unsophisticated. In most cases, nothing more than a vacuum bag and heating blanket would be available.
The repaired structure must exhibit performance properties such as tensile strength, impact resistance, shear modulus, etc. on a par with the original materials to which they are mated, to preserve integrity of the repaired article. Generally, the original materials will be special factory preparations, involving the use of sophisticated resin blends and curing profiles, including cure temperatures of 350.degree. F and more for conventional epoxy systems.
The repair system must also be susceptible of easy and rapid use by relatively unsophisticated personnel. Component mixing, sophisticated application procedures and the like, which may be acceptable in factories devoted to the manufacture of high cost articles, will not be practical.
The repair systems currently available generally fall under two classes, neither of which satisfactorily meets the above objectives. The bolting or mechanical fastening of metal or cured composite material patches over the damaged area is impractical and does not yield satisfactory results. This process can frequently be difficult or impossible on-site due to lack of backside access, and the use of blind fasteners and drilling operations induce added stresses and possible areas of failure to the repair site.
The alternative conventional system involves the use of a wet lay-up procedure. This employs, as a patching material, a two part system that must be mixed prior to repair and used immediately. While the wet lay-up system does offer long term shelf life, and simply cure profiles, the control of material mixing, resin to fiber ratio and wet out of fiber frustrates attempts to reliably achieve the desired properties. This system also involves an increased exposure of workers to hazardous and irritating chemicals, including amines and epoxy components. Further, due to the need to mix the two components of the system, there is a high potential for error during compounding, with the resulting risk of an exothermic reaction, especially when large batches are required.
Accordingly, it remains a pressing need of the industry to provide a resin matrix fiber reinforced composite material repair system, meeting all of the above objectives.